Haskell Foldable Wats

Chris Allen cma at bitemyapp.com
Wed Feb 24 16:14:59 UTC 2016

Aren't these sorts of mistakes impossible with ML sigs and structs?

On 02/24/2016 10:08 AM, Augustsson, Lennart wrote:
> Having length of a tuple not be a type error is not only confusing for 
> beginners.
> It can also mask bugs in real code written by experts.  And it’s 
> already happened.
> Since length of a tuple returning 1 is nonsensical and useless, it 
> should simply not be legal.
> *From:*Libraries [mailto:libraries-bounces at haskell.org] *On Behalf Of 
> *Nathan Bouscal
> *Sent:* 24 February 2016 15:56
> *To:* Kosyrev Serge
> *Cc:* Haskell Libraries
> *Subject:* Re: Haskell Foldable Wats
> On Wed, Feb 24, 2016 at 3:49 PM, Kosyrev Serge 
> <_deepfire at feelingofgreen.ru <mailto:_deepfire at feelingofgreen.ru>> wrote:
> Nathan Bouscal <nbouscal at gmail.com <mailto:nbouscal at gmail.com>> writes:
> > Apologies for being unclear. By marginal, I meant that I expect many
> > learners who end up confused by this would have otherwise come across
> > the same confusion by other paths.
> This sounds odd to me in two ways:
> 1. it is not "the same" confusion -- it is a worse confusion, for
>    several reasons:
>    - the questionable Functor instance gives a cognitive dissonance for
>      not transforming the entirety of the tuple (for good reasons, but 
> still!)
>    - the questionable Functor is a puzzlingly arbitrary choice among a
>      N-family of options that an N-tuple provides.
> As Ed pointed out, there is nothing arbitrary about the choice. This 
> actually makes tuples in some sense a uniquely good way of explaining 
> how things work, because you can show why this restriction exists.
>     2. but even if the confusion itself wasn't made worse -- its
>     impact has
>        increased, because exposure to it has increased
> To the extent that exposure has increased, I would think it would be 
> caused by confusion of the "lists vs tuples" type, which seems 
> important to resolve early. That resolution doesn't really need to say 
> anything about how Functor works, and can instead be more along the 
> lines of "What problem were you trying to solve by taking the length 
> of a tuple? Should you have been using a list instead?"
>     --
>     с уважениeм / respectfully,
>     Косырев Сергей
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